ORISKANY SANDSTONE. 
465 
LAMELL1BRANCHIATA OF THE ORISKANY SANDSTONE. 
The lamellibranchiate shells of this rock comprise few species, and these 
usually occur in such condition as to afford very unsatisfactory means of 
determination ; the specimens being for the most part in the form of casts 
of the interior, or impressions of the exterior of the shell. 
Avicula textilis, var. areuaria. 
Plate CIX. Fig. 1 & 2; and Plate CX. Fig. 2. 
Avicula textilis : This volume, page 288. 
Shell large, obliquely subovate; the proportions of length and height 
variable. Left valve becoming moderately and regularly convex from 
the base, the greatest convexity being about the first third below the 
hinge-line. Posterior wing large, extending along the margin of the 
body of the shell halfway from beak to base. Anterior wing small, 
triangular, wrinkled. The right valve is slightly concave, smaller than 
the other, faintly marked by the radiating ribs, which sometimes are 
scarcely seen. 
Surface marked by strong radiating ribs sometimes regularly dichotomizing 
and subequal, and in other specimens quite unequal, showing a few 
stronger ribs with several finer ones between, and these are crossed by 
strongly elevated imbricating lamellm. 
This is the common form of Avicula in the Oriskany sandstone, and often attains 
a height of four inches or more. Although presenting considerable variety in its 
aspect and surface markings, I am unable at the present time to point out reliable 
specific distinctions between this one and the species of the Lower Helderberg 
limestones. 
PLATE CIX. 
Fig. 1. An impression made in sandstone by the exterior surface of the smaller or right 
valve. 
Fig. 2. The exterior imprint of the left valve of this species. 
[ Palaeontology III.] 59 
